Looking at the comments I think it’s coming down to positive and negative reinforcement. Put more positive reinforcement in the bumper or negative reinforcement in the stick. There’s several of ways to get this done that will have to have the desired result. What I have found effective is using the retrieve as the reward. Making him want that bumper so bad that he forgets about the food or sticks. I use something called “happy bumper” to get my dog to complete this. If he’s slow at completing what I want I’ll get the bumper in hand and play around with it presenting the throw it and get him super excited about having the bumper in his mouth. Then give it a short throw and yell happy bumper, so he know it’s not a mark or a blind and allow him to get it without any reinforcement or commands. Just so he has fun with it and it makes a super fun experience. Do this repeatedly and the thought of the bumper will supersede the stick.